1. Field of the Invention
The present invention generally relates to a method of forming a semiconductor device, and more particularly to a method for increasing a capacitor size on bulk and silicon-on-insulator (SOI) wafers.
2. Description of the Related Art
Large capacitors are usually needed for integrated circuits such as decoupling capacitors for a power or signal bus stabilization, or capacitors for analog application circuits such as reservoirs for a charge pump. However, it is very difficult to implement a large capacitor in a chip where the chip xe2x80x9creal estatexe2x80x9d is already very limited.
Therefore, many methods have been proposed to increase the capacitor value without increasing the chip area. A first method is to form a deep silicon island capacitor. Making an array of deep silicon island capacitors where the capacitors are similar to those used in the DRAM used for cell node data storage. This process of making deep silicon island capacitors is very expensive, unless DRAM array is built on the same chip. However, for other chips such as SOI, or silicon-on-oxide, due to the existing buried oxide layer, if the only goal is to make a large capacitor using a deep silicon island structure, the processing and other costs will be prohibitive and thus impractical.
Another method is to include high dielectric constant material in the capacitor (e.g., between the two conductive portions or plates). This also increases the cost of processing significantly. That is, special materials and extra process steps must be employed.
Yet another method is to form a stack capacitor. However, this technique makes planarization and interconnection more difficult. Thus, this approach also is not a good option in making a large capacitor.
Other methods, such as a xe2x80x9croughened surfacexe2x80x9d capacitor, etc. are equally impractical and require special techniques to implement. For example, in the roughed surface approach, typically polysilicon is grown under special conditions (e.g., at a specific temperature) to form an irregular surface area (e.g., having semispherical bumps). Such semispherical bumps increase the surface area.
In another technique, for example, silicon and germanium may grow to form a rough conductive layer. This conductive layer is then conformally coated with a dielectric. This technique also increases the surface (and thus capacitance) of the structure. However, as mentioned above, such techniques are impractical and are not easily implemented. Further, the capacitance enhancement is not reproducible, and therefore not suitable especially for some analog applications where the capacitor size must be precisely controlled.
Thus, a problem arises regarding how to build large capacitors with good area control (e.g., providing a specific capacitance) on a chip, without extra process cost.
In view of the foregoing and other problems of the conventional methods and structures, an object of the present invention is to provide a method and structure in which a large capacitor can be built compatible with complementary metal oxide semiconductor (CMOS) devices on bulk or a silicon-on-insulator (SOI) wafer.
Another object is to provide a method of forming a low-cost capacitor, without any additional processing costs.
Yet another object is to use a phase-shift mask technique to increase the capacitor density to reduce pitch by patterning the sub-lithographic feature by intentionally increasing the ratio of silicon width to spacing between adjacent islands.
In a first aspect of the present invention, a method of forming a semiconductor structure, includes forming at least one conductive island having a predetermined sidewall angle in a conductive substrate, forming a dielectric material over the at least one conductive island, forming a conductive material over the dielectric material, and forming a contact to the conductive material and the at least one conductive island.
With the unique and unobvious features of the invention, a low cost method is provided to form a high performance capacitor. Further, the present invention is useful for both bulk and SOI substrates. Additionally, the technique is especially useful for SOI substrates with a thick silicon layer (e.g., having a thickness greater than 0.5 xcexcm).
Another advantage of the invention is that large capacitors are built simultaneously with a high performance transistor or a transistor with a wrap-around gate structure.